Numerous types of mass flow meters are generally available. Some are based on viscous drag, others employ the Venturi effect to measure a pressure drop, still others use positive displacement either rotary or linear. For low flow rates, local heating of a passing fluid and a variation in the resistance of a resistive element or the current/voltage required to maintain a constant temperature/current in the resistive element is conventionally used to determine fluid flow rate, wherein the fluid can be either a liquid or a gas. Rudent et al. in U.S. Pat. No. 6,354,150 “Sensor for a Capillary Tube of a Mass Flow Meter”, issued Mar. 12, 2002, describe a mass flow rate sensor based on the time-of-flight of a thermal pulse. The time-of-flight of a small volume of liquid is measured from an upstream position where the temperature is modified to a downstream detector. However, the thermal sensor requires specially insulated components to avoid thermal transients, which can be a disadvantage.
Wu and Sansen (Wu, J. and Sansen, W. Electrochemical time of flight flow sensor, Sensors and Actuators, 97, 68–74, Apr. 1, 2002) describe a time of flight flow sensor for measuring the flow rates of aqueous fluids in the range of 1–15 μL/min. The device uses an in situ electrochemically produced tracer to determine fluid flow rate and is comprised of two electrochemical cells integrated in the flow channel. An upstream electrochemical cell functions as an oxygen producer and a downstream electrochemical cell functions as an amperometric oxygen detector. Oxygen produced at the upstream cell by an electrochemical pulse is detected at the downstream amperometric cell and the flow rate is determined by the time difference between the two pulses. However, this device suffers from the limitation that it is unable to measure fluid flow rates less than about 1 μL/min (1000 nL/min) due to limitations of the oxygen sensor (ibid., p. 71).